1. Field of the Invention
This invention relates to ferroelectric materials and memories utilizing such materials, and more particularly to such a material and memory which can be made at relatively low temperatures.
2. Statement of the Problem
It has been known since at least the 1950""s that if a practical ferroelectric memory could be made, it would provide a fast, dense, non-volatile memory that could be operated at relatively low voltages. See Orlando Auciello et al., xe2x80x9cThe Physics of Ferroelectric Memoriesxe2x80x9d, Physics Today, Jul. 1998, pp. 22-27. Such memories are based on the fact that ferroelectrics can be polarized when a voltage is placed across them. By reversing the direction of the voltage, the polarization can be reversed, which provides a bistable electronic element that is ideally suited for digital electronics. The non-votatility derives from the fact that the polarization remains after the voltage is removed.
For years, the objective of making such a ferroelectric memory was blocked by the fact that all known ferroelectrics fatigued; that is, the polarizability decreased by a factor of two or more as the ferroelectric was switched. In 1991, a material, now called a layered superlattice material, was discovered which did not fatigue. See, U.S. Pat. No. 5,519,234 issued on May 21, 1996 to Araujo et al. Because this was a new material in the integrated circuit fabrication industry, and because ferroelectrics memory architecture was relatively undeveloped as compared to, for example, dynamic random access memories (DRAMs) and Flash memories, it has taken about nine years to solve all the problems that need to be solved to produce a memory that would be competitive with conventional memories. There remains yet one significant problem: ferroelectrics are ceramics, and it requires relatively high heat, on the order of 650xc2x0 C. to 850xc2x0 C., to crystallize them. Conventional memory materials, such as doped silicon, silicon dioxide, aluminum, tungsten, titanium, etc. are somewhat sensitive to temperature, and thus integration of the ferroelectrics into the conventional processes remains problematic.
Numerous attempts have been made to overcome the temperature problem. See, U.S. patent application Ser. No. 5,508,226 issued on Apr. 16, 1996 to Takeshi Ito et al., U.S. Pat. No. 6,133,092 issued Oct. 17, 2000 to Shinichiro Hayashi et al., and U.S. Pat. No. 5,962,069 issued Oct. 5, 1999 to Gunther Shindler et al. While the thermal budget to which the partially completed memory is exposed has been significantly lowered by these advances, heat still remains the principle hurdle to the high yields that are necessary to make ferroelectric memories as economical to manufacture as memories such as DRAMs.
The invention solves the above problem by providing a composite ferroelectric material that also has good ferroelectric performance. The material is a composite of a ferroelectric component and a dielectric component, which is preferably a fluxor.
The invention preferably comprises a ferroelectric integrated circuit memory which includes a ferroelectric thin film comprising a ferroelectric component and a fluxor. The fluxor is a material that has a higher crystallization velocity than the ferroelectric component. It is possible the fluxor is another ferroelectric with a higher crystallization velocity than the principal ferroelectric component, but preferably the fluxor is a dielectric material, and most preferably a single metal oxide or a solid solution of a plurality of single metal oxides. The fluxor can include some carbonate that carries through from the precursor, but the amount of carbon should be limited.
The invention also provides a precursor for making a composite ferroelectric material. The precursor preferably comprises a metalorganic compound in a solvent, the metal organic compound including metals in the amounts required to form the above-mentioned composite ferroelectric material upon heating of the precursor.
Preferably, the composite ferroelectric comprises a microscopic mixture of ferroelectric domains and dielectric domains. The ferroelectric domains and dielectric domains are preferably distributed essentially homogeneously distributed in the material. Thus the material comprises small ferroelectric crystals separated by a dielectric medium.
The invention also provides a low temperature method of making a ferroelectric thin film. The method comprises providing a metal organic compound including metals in the proportions required to form a microscopically composite ferroelectric/fluxor material.
The invention not only provides a ferroelectric material and memory which can be made at low temperatures, but also provides such a material and memory with excellent electronic properties, and which is economical to manufacture. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.